Annals of Mathematics Topological equivalence of linear representations for cyclic groups: I By Ian Hambleton and Erik K. Pedersen Annals of Mathematics, 161 (2005), 61–104 Topological equivalence of linear representations for cyclic groups: I By Ian Hambleton and Erik K. Pedersen* Abstract In the two parts of this paper we prove that the Reidemeister torsion invariants determine topological equivalence of G-representations, for G a finite cyclic group. 1. Introduction Let G be a finite group and V , V  finite dimensional real orthogonal rep- resentations of G. Then V is said to be topologically equivalent to V  (denoted V ∼ t V  ) if there exists a homeomorphism h: V → V  which is G-equivariant. If V , V  are topologically equivalent, but not linearly isomorphic, then such a homeomorphism is called a nonlinear similarity. These notions were intro- duced and studied by de Rham [31], [32], and developed extensively in [3], [4], [22], [23], and [8]. In the two parts of this paper, referred to as [I] and [II], we complete de Rham’s program by showing that Reidemeister torsion invariants and number theory determine nonlinear similarity for finite cyclic groups. A G-representation is called free if each element 1 = g ∈ G fixes only the zero vector. Every representation of a finite cyclic group has a unique maximal free subrepresentation. Theorem. Let G be a finite cyclic group and V 1 , V 2 be free G-represen- tations. For any G-representation W , the existence of a nonlinear similarity V 1 ⊕W ∼ t V 2 ⊕W is entirely determined by explicit congruences in the weights of the free summands V 1 , V 2 , and the ratio ∆(V 1 )/∆(V 2 ) of their Reidemeister torsions, up to an algebraically described indeterminacy. *Partially supported by NSERC grant A4000 and NSF grant DMS-9104026. The authors also wish to thank the Max Planck Institut f¨ur Mathematik, Bonn, for its hospitality and support. 62 IAN HAMBLETON AND ERIK K. PEDERSEN The notation and the indeterminacy are given in Section 2 and a detailed statement of results in Theorems A–E. For cyclic groups of 2-power order, we obtain a complete classification of nonlinear similarities (see Section 11). In [3], Cappell and Shaneson showed that nonlinear similarities V ∼ t V  exist for cyclic groups G = C(4q) of every order 4q  8. On the other hand, if G = C(q)orG = C(2q), for q odd, Hsiang-Pardon [22] and Madsen- Rothenberg [23] proved that topological equivalence of G-representations im- plies linear equivalence (the case G = C(4) is trivial). Since linear G-equivalence for general finite groups G is detected by restriction to cyclic subgroups, it is reasonable to study this case first. For the rest of the paper, unless otherwise mentioned, G denotes a finite cyclic group. Further positive results can be obtained by imposing assumptions on the isotropy subgroups allowed in V and V  . For example, de Rham [31] proved in 1935 that piecewise linear similarity implies linear equivalence for free G-representations, by using Reidemeister torsion and the Franz Independence Lemma. Topological invariance of Whitehead torsion shows that his method also rules out nonlinear similarity in this case. In [17, Th. A] we studied “first- time” similarities, where Res K V ∼ = Res K V  for all proper subgroups K  G, and showed that topological equivalence implies linear equivalence if V , V  have no isotropy subgroup of index 2. This result is an application of bounded surgery theory (see [16], [17, §4]), and provides a more conceptual proof of the Odd Order Theorem. These techniques are extended here to provide a neces- sary and sufficient condition for nonlinear similarity in terms of the vanishing of a bounded transfer map (see Theorem 3.5). This gives a new approach to de Rham’s problem. The main work of the present paper is to establish meth- ods for effective calculation of the bounded transfer in the presence of isotropy groups of arbitrary index. An interesting question in nonlinear similarity concerns the minimum possible dimension for examples. It is easy to see that the existence of a nonlinear similarity V ∼ t V  implies dim V = dim V   5. Cappell, Shaneson, Steinberger and West [8] proved that 6-dimensional similarities exist for G = C(2 r ), r  4 and referred to the 1981 Cappell-Shaneson preprint (now pub- lished [6]) for the complete proof that 5-dimensional similarities do not exist for any finite group. See Corollary 9.3 for a direct argument using the criterion of Theorem A in the special case of cyclic 2-groups. In [4], Cappell and Shaneson initiated the study of stable topological equvalence for G-representations. We say that V 1 and V 2 are stably topologi- cally similar (V 1 ≈ t V 2 ) if there exists a G-representation W such that V 1 ⊕ W ∼ t V 2 ⊕ W . Let R Top (G)=R(G)/R t (G) denote the quotient group of the real representation ring of G by the subgroup R t (G)= {[V 1 ] − [V 2 ] | V 1 ≈ t V 2 }. In [4], R Top (G) ⊗ Z[1/2] was computed, and the torsion subgroup was shown to be 2-primary. As an application of our general SIMILARITIES OF CYCLIC GROUPS: I 63 results, we determine the structure of the torsion in R Top (G), for G any cyclic group (see [II, §13]). In Theorem E we give the calculation of R Top (G) for G = C(2 r ). This is the first complete calculation of R Top (G) for any group that admits nonlinear similarities. Contents 1. Introduction 2. Statement of results 3. A criterion for nonlinear similarity 4. Bounded R − transfers 5. Some basic facts in K- and L-theory 6. The computation of L p 1 (ZG, w) 7. The proof of Theorem A 8. The proof of Theorem B 9. Cyclic 2-Groups: preliminary results 10. The proof of Theorem E 11. Nonlinear similarity for cyclic 2-groups References 2. Statement of results We first introduce some notation, and then give the main results. Let G = C(4q), where q>1, and let H = C(2q) denote the subgroup of index 2 in G. The maximal odd order subgroup of G is denoted G odd . We fix a generator G = t and a primitive 4q th -root of unity ζ = exp 2πi/4q. The group G has both a trivial 1-dimensional real representation, denoted R + , and a nontrivial 1-dimensional real representation, denoted R − . A free G-representation is a sum of faithful 1-dimensional complex repre- sentations. Let t a , a ∈ Z, denote the complex numbers C with action t·z = ζ a z for all z ∈ C. This representation is free if and only if (a, 4q) = 1, and the coeffi- cient a is well-defined only modulo 4q. Since t a ∼ = t −a as real G-representations, we can always choose the weights a ≡ 1 mod 4. This will be assumed unless otherwise mentioned. Now suppose that V 1 = t a 1 + ··· + t a k is a free G-representation. The Reidemeister torsion invariant of V 1 is defined as ∆(V 1 )= k  i=1 (t a i − 1) ∈ Z[t]/{±t m } . 64 IAN HAMBLETON AND ERIK K. PEDERSEN Let V 2 = t b 1 + ···+ t b k be another free representation, such that S(V 1 ) and S(V 2 ) are G-homotopy equivalent. This just means that the products of the weights  a i ≡  b i mod 4q. Then the Whitehead torsion of any G-homotopy equivalence is determined by the element ∆(V 1 )/∆(V 2 )=  (t a i − 1)  (t b i − 1) since Wh(ZG) → Wh(QG) is monic [26, p. 14]. When there exists a G-homotopy equivalence f : S(V 2 ) → S(V 1 ) which is freely G-normally cobor- dant to the identity map on S(V 1 ), we say that S(V 1 ) and S(V 2 ) are freely G-normally cobordant. More generally, we say that S(V 1 ) and S(V 2 ) are s-normally cobordant if S(V 1 ⊕ U) and S(V 2 ⊕ U) are freely G-normally cobor- dant for all free G-representations U. This is a necessary condition for non- linear similarity, which can be decided by explicit congruences in the weights (see [35, Th. 1.2] and [II, §12]). This quantity, ∆(V 1 )/∆(V 2 ) is the basic invariant determining nonlinear similarity. It represents a unit in the group ring ZG, explicitly described for G = C(2 r ) by Cappell and Shaneson in [5, §1] using a pull-back square of rings. To state concrete results we need to evaluate this invariant modulo suitable indeterminacy. The involution t → t −1 induces the identity on Wh(ZG), so we get an element {∆(V 1 )/∆(V 2 )}∈H 0 (Wh(ZG)) where we use H i (A) to denote the Tate cohomology H i (Z/2; A)ofZ/2 with coefficients in A. Let Wh(ZG − ) denote the Whitehead group Wh(ZG) together with the involution induced by t →−t −1 . Then for τ(t)=  (t a i −1)  (t b i −1) , we compute τ(t)τ(−t)=  (t a i − 1)  ((−t) a i − 1)  (t b i − 1)  ((−t) b i − 1) =  (t 2 ) a i − 1 ((t 2 ) b i − 1) which is clearly induced from Wh(ZH). Hence we also get a well defined element {∆(V 1 )/∆(V 2 )}∈H 1 (Wh(ZG − )/ Wh(ZH)) . This calculation takes place over the ring Λ 2q = Z[t]/(1 + t 2 + ···+ t 4q−2 ), but the result holds over ZG via the involution-invariant pull-back square ZG → Λ 2q ↓↓ Z[Z/2] → Z/2q[Z/2] Consider the exact sequence of modules with involution: K 1 (ZH) → K 1 (ZG) → K 1 (ZH →ZG) →  K 0 (ZH) →  K 0 (ZG)(2.1) SIMILARITIES OF CYCLIC GROUPS: I 65 and define Wh(ZH → ZG)=K 1 (ZH → ZG)/ {±G} . We then have a short exact sequence 0 → Wh(ZG)/ Wh(ZH) → Wh(ZH →ZG) → k → 0 where k = ker(  K 0 (ZH) →  K 0 (ZG)). Such an exact sequence of Z/2-modules induces a long exact sequence in Tate cohomology. In particular, we have a coboundary map δ : H 0 (k) → H 1 (Wh(ZG − )/ Wh(ZH)) . Our first result deals with isotropy groups of index 2, as is the case for all the nonlinear similarities constructed in [3]. Theorem A. Let V 1 = t a 1 + ···+ t a k and V 2 = t b 1 + ···+ t b k be free G-representations, with a i ≡ b i ≡ 1mod4. There exists a topological similarity V 1 ⊕ R − ∼ t V 2 ⊕ R − if and only if (i)  a i ≡  b i mod 4q, (ii) Res H V 1 ∼ = Res H V 2 , and (iii) the element {∆(V 1 )/∆(V 2 )}∈H 1 (Wh(ZG − )/ Wh(ZH)) is in the image of the coboundary δ : H 0 (k) → H 1 (Wh(ZG − )/ Wh(ZH)). Remark 2.2. The condition (iii) simplifies for G a cyclic 2-group since H 0 (k) = 0 in that case (see Lemma 9.1). Theorem A should be compared with [3, Cor.1], where more explicit conditions are given for “first-time” similarities of this kind under the assumption that q is odd, or a 2-power, or 4q is a “tempered” number. See also [II, Th. 9.2] for a more general result concerning similarities without R + summands. The case dim V 1 = dim V 2 = 4 gives a reduction to number theory for the existence of 5-dimensional similarities (see Remark 7.2). Our next result uses a more elaborate setting for the invariant. Let Φ=   ZH →  Z 2 H ↓↓ ZG →  Z 2 G   and consider the exact sequence 0 → K 1 (ZH →ZG) → K 1 (  Z 2 H →  Z 2 G) → K 1 (Φ) →  K 0 (ZH →ZG) → 0 . (2.3) Again we can define the Whitehead group versions by dividing out trivial units {±G}, and get a double coboundary δ 2 : H 1 (  K 0 (ZH →ZG − )) → H 1 (Wh(ZH →ZG − )) . 66 IAN HAMBLETON AND ERIK K. PEDERSEN There is a natural map H 1 (Wh(ZG − )/ Wh(ZH)) → H 1 (Wh(ZH → ZG − )), and we will use the same notation {∆(V 1 )/∆(V 2 )} for the image of the Reidemeister torsion invariant in this new domain. The nonlinear similari- ties handled by the next result have isotropy of index  2. Theorem B. Let V 1 = t a 1 + ···+ t a k and V 2 = t b 1 + ···+ t b k be free G-representations. There exists a topological similarity V 1 ⊕ R − ⊕ R + ∼ t V 2 ⊕ R − ⊕ R + if and only if (i)  a i ≡  b i mod 4q, (ii) Res H V 1 ∼ = Res H V 2 , and (iii) the element {∆(V 1 )/∆(V 2 )} is in the image of the double coboundary δ 2 : H 1 (  K 0 (ZH →ZG − )) → H 1 (Wh(ZH →ZG − )) . This result can be applied to 6-dimensional similarities. Corollary 2.4. Let G = C(4q), with q odd, and suppose that the fields Q(ζ d ) have odd class number for all d | 4q. Then G has no 6-dimensional nonlinear similarities. Remark 2.5. For example, the class number condition is satisfied for q  11, but not for q = 29. The proof is given in [II, §11]. This result corrects [8, Th. 1(i)], and shows that the computations of R Top (G) given in [8, Th. 2] are incorrect. We explain the source of these mistakes in Remark 6.4. Our final example of the computation of bounded transfers is suitable for determining stable nonlinear similarities inductively, with only a minor as- sumption on the isotropy subgroups. To state the algebraic conditions, we must again generalize the indeterminacy for the Reidemeister torsion invari- ant to include bounded K-groups (see [II, §5]). In this setting  K 0 (ZH → ZG)=  K 0 (C R − ,G (Z)) and Wh(ZH →ZG) = Wh(C R − ,G (Z)). We consider the analogous double coboundary δ 2 : H 1 (  K 0 (C W ×R − ,G (Z))) → H 1 (Wh(C W ×R − ,G (Z))) and note that there is a map Wh(C R − ,G (Z) → Wh(C W ×R − ,G (Z)) induced by the inclusion on the control spaces. We will use the same notation {∆(V 1 )/∆(V 2 )} for the image of our Reidemeister torsion invariant in this new domain. Theorem C. Let V 1 = t a 1 + ···+ t a k and V 2 = t b 1 + ···+ t b k be free G-representations. Let W be a complex G-representation with no R + sum- mands. Then there exists a topological similarity V 1 ⊕ W ⊕ R − ⊕ R + ∼ t V 2 ⊕ W ⊕ R − ⊕ R + if and only if SIMILARITIES OF CYCLIC GROUPS: I 67 (i) S(V 1 ) is s-normally cobordant to S(V 2 ), (ii) Res H (V 1 ⊕ W ) ⊕ R + ∼ t Res H (V 2 ⊕ W ) ⊕ R + , and (iii) the element {∆(V 1 )/∆(V 2 )} is in the image of the double coboundary δ 2 : H 1 (  K 0 (C W max ×R − ,G (Z))) → H 1 (Wh(C W max ×R − ,G (Z))) , where 0 ⊆ W max ⊆ W is a complex subrepresentation of real dimension  2, with maximal isotropy group among the isotropy groups of W with 2-power index. Remark 2.6. The existence of a similarity implies that S(V 1 ) and S(V 2 ) are s-normally cobordant. In particular, S(V 1 ) must be freely G-normally cobordant to S(V 2 ) and this unstable normal invariant condition is enough to give us a surgery problem. The computation of the bounded transfer in L-theory leads to condition (iii), and an expression of the obstruction to the existence of a similarity purely in terms of bounded K-theory. To carry out this computation we may need to stabilize in the free part, and this uses the s-normal cobordism condition. Remark 2.7. Theorem C is proved in [II, §9]. Note that W max =0in condition (iii) if W has no isotropy subgroups of 2-power index. Theorem C suffices to handle stable topological similarities, but leaves out cases where W has an odd number of R − summands (handled in [II, Th. 9.2] and the results of [II, §10]). Simpler conditions can be given when G = C(2 r ) (see §9 in this part, [I]). The double coboundary in (iii) can also be expressed in more “classical” terms by using the short exact sequence 0 → Wh(C R − ,G (Z)) → Wh(C W max ×R − ,G (Z)) → K 1 (C >R − W max ×R − ,G (Z)) → 0 (2.8) derived in [II, Cor. 6.9]. We have K 1 (C >R − W max ×R − ,G (Z)) = K −1 (ZK), where K is the isotropy group of W max , and Wh(C R − ,G (Z)) = Wh(ZH → ZG). The indeterminacy in Theorem C is then generated by the double coboundary δ 2 : H 1 (  K 0 (ZH →ZG − )) → H 1 (Wh(ZH →ZG − )) used in Theorem B and the coboundary δ : H 0 (K −1 (ZK)) → H 1 (Wh(ZH →ZG − )) from the Tate cohomology sequence of (2.8). Finally, we will apply these results to R Top (G). In Part [II, §3], we will define a subgroup filtration R t (G) ⊆ R n (G) ⊆ R h (G) ⊆ R(G)(2.9) 68 IAN HAMBLETON AND ERIK K. PEDERSEN on the real representation ring R(G), inducing a filtration on R Top (G)=R(G)/R t (G) . Here R h (G) consists of those virtual elements with no homotopy obstruction to similarity, and R n (G) the virtual elements with no normal invariant obstruction to similarity (see [II, §3] for more precise definitions). Note that R(G) has the nice basis {t i ,δ,ε | 1  i  2q − 1}, where δ =[R − ] and ε =[R + ]. Let R free (G)={t a | (a, 4q)=1}⊂R(G) be the subgroup generated by the free representations. To complete the definition, we let R free (C(2)) = {R − } and R free (e)={R + }. Then R(G)=  K⊆G R free (G/K) and this direct sum splitting intersected with the filtration above gives the sub- groups R free h (G), R free n (G) and R free t (G). In addition, we can divide out R free t (G) and obtain subgroups R free h,Top (G) and R free n,Top (G)ofR free Top (G)=R free (G)/R free t (G). By induction on the order of G, we see that it suffices to study the summand R free Top (G). Let  R free (G) = ker(Res: R free (G) → R free (G odd )), and then project into R Top (G) to define  R free Top (G)=  R free (G)/R free t (G) . In [II, §4] we prove that  R free Top (G)isprecisely the torsion subgroup of R free Top (G), and in [II, §13] we show that the subquotient  R free n,Top (G)=  R free n (G)/R free t (G) always has exponent two. Here is a specific computation (correcting [8, Th. 2]), proved in [II, §13]. Theorem D. Let G = C(4q), with q>1 odd, and suppose that the fields Q(ζ d ) have odd class number for all d | 4q. Then  R free Top (G)=Z/4 generated by (t − t 1+2q ). For any cyclic group G, both R free (G)/R free h (G) and R free h (G)/R free n (G) are torsion groups which can be explicitly determined by congruences in the weights (see [II, §12] and [35, Th. 1.2]). We conclude this list of sample results with a calculation of R Top (G) for cyclic 2-groups. Theorem E. Let G = C(2 r ), with r  4. Then  R free Top (G)=  α 1 ,α 2 , ,α r−2 ,β 1 ,β 2 , ,β r−3  subject to the relations 2 s α s =0for 1  s  r − 2, and 2 s−1 (α s + β s )=0for 2  s  r − 3, together with 2(α 1 + β 1 )=0. SIMILARITIES OF CYCLIC GROUPS: I 69 The generators for r  4 are given by the elements α s = t − t 5 2 r−s−2 and β s = t 5 − t 5 2 r−s−2 +1 . We remark that  R free Top (C(8)) = Z/4 is generated by t − t 5 . In Theorem 11.6 we use this information to give a complete topological classification of linear representations for cyclic 2-groups. Acknowledgement. The authors would like to express their appreciation to the referee for many constructive comments and suggestions. 3. A criterion for nonlinear similarity Our approach to the nonlinear similarity problem is through bounded surgery theory (see [11], [16], [17]): first, an elementary observation about topological equivalences for cyclic groups. Lemma 3.1. If V 1 ⊕W ∼ t V 2 ⊕W  , where V 1 , V 2 are free G-representations, and W and W  have no free summands, then there is a G-homeomorphism h: V 1 ⊕ W → V 2 ⊕ W such that h    1=H≤G W H is the identity. Proof. Let h be the homeomorphism given by V 1 ⊕ W ∼ t V 2 ⊕ W  .We will successively change h, stratum by stratum. For every subgroup K of G, consider the homeomorphism of K-fixed sets h K : W K → W K . This is a homeomorphism of G/K, hence of G-representations. As G-represen- tations we can split V 2 ⊕ W  = U ⊕ W K ∼ t U ⊕ W K = V 2 ⊕ W  where the similarity uses the product of the identity and (h K ) −1 . Notice that the composition of h with this similarity is the identity on the K-fixed set. Rename W  as W  and repeat this successively for all subgroups. We end up with W = W  and a G-homeomorphism inducing the identity on the singular set. One consequence is Lemma 3.2. If V 1 ⊕ W ∼ t V 2 ⊕ W , then there exists a G-homotopy equiv- alence S(V 2 ) → S(V 1 ). [...]... 5-dimensional nonlinear similarities Proof The Reidemeister torsion quotients for possible 5-dimensional similarities are represented by the units U1 ,i which form a basis of H 1 (Wh(ZG− )) (see [5], [8, p 733]) Higher-dimensional similarities of cyclic 2-groups were previously studied in the 1980’s The 6-dimensional case was worked out in detail for cyclic 2-groups in [8], and general conditions A–D were... (Z)) 0 II II II II I$ (8.1) u: uu uu uu uu II II II II I$ u: uu uu uu uu u: uu uu uu uu II II II II I$ u: uu uu uu uu II II II II I$ H 1 (∆R− ) Lh (CR− ,G (Z)) 1 Ls (CR− ,G (Z)) 1 Lp (CR− ,G (Z)) 1 8 H 0 (K0 (CR− ,G (Z))) 8 where H 1 (∆R− ) denotes the relative group of the double coboundary map Notice that some of the groups in this diagram already appeared as relative L-groups in the last section We... the image of H 1 (Wh(ZG− )/ Wh(ZH)) in the relative group H 1 (∆R− ) This sharper version will be used in determining the nonlinear similarities for cyclic 2-groups Lemma 8.2 Let W be a complex G-representation, with W G = 0, containing all the nontrivial irreducible representations of G with isotropy of 2-power index (i) Automorphisms of G induce the identity on the image of H 1 (Wh(ZG− )/ Wh(ZH)) in... proof of Theorem A The condition (i) is equivalent to assuming that S(V1 ) and S(V2 ) are freely G-homotopy equivalent Condition (ii) is necessary by Corollary 3.10 which rules out nonlinear similarities of semifree representations Condition (ii) also implies that S(V1 ) is s-normally cobordant to S(V2 ) by [3, Prop 2.1], which is another necessary condition for topological similarity Thus under conditions... gives necessary and sufficient conditions for the existence of 5-dimensional similarities Consider the situation in dimensions 5 By character theory it suffices to consider a cyclic group G, which must be of order divisable by 4, say 4q, with index 2 subgroup H It suffices by Lemma 3.1 to consider the following situation V1 ⊕ W ∼t V2 ⊕ W where Vi are free homotopy equivalent representations which become isomorphic... OF CYCLIC GROUPS: I The nonexistence of a 6-dimensional similarity in Part (iv) follows from the calculation [8, Cor (iii), p 719] Theorem 9.4 shows that there is no higher dimensional similarity Part (v) is again immediate, and Part (vi) is an easy calculation showfree ing that the element 2s−2 (αs (r) − βs (r)) ∈ Rh (G) fails the first congruence condition in [35, Th 1.2], which for this case is the... Proof For necessity, we refer the reader to [17] where this is proved using a version of equivariant engulfing For sufficiency, we notice that crossing with SIMILARITIES OF CYCLIC GROUPS: I 71 R gives an isomorphism of the bounded surgery exact sequences parametrized by W to simple bounded surgery exact sequences parametrized by W × R By the bounded s-cobordism theorem, this means that the vanishing of. .. 4] given by the subgroup inclusion induces an injection on this subgroup ˆ Im((5 − γ)ψn ) However the map Ind corresponds under the identification in [25, Th 2.4] i+ 1 with the explicit map ind(ei ) = i ei+1 (see last paragraph of [25, §2]) Using ˆ this explicit formula, we need to check that x ∈ ker(ψn+1 ◦ ind) implies that n ˆ x ∈ ker ψn Suppose that x = i= 0 ai ei ∈ M n Then   n n i+ 1 n+1 ai 2i+ 1... split short exact at each level The K-theory of this category is the same as the K-theory of CV,G (R) For an argument working in this generality see [9] Tensoring with the chain complex of (V, G) induces a map of categories with cofibrations and weak equivalences, hence a map on K-theory It is elementary to see that this agrees with the geometric definition in low dimensions, since identification of the... + i + 1; i + 1, 2r−s−2 + i) also represents a unit in ZG Since dim Vi = 4 the spheres S(V1 ) and S(V2 ) are G-normally cobordant Then we restrict to H and use induction on r, starting with r = 4 where the similarity follows from [8, Cor (iii),p 719] Theorem 9.4 and Galois invariance of the surgery obstruction under the action of σ (by Lemma 8.2(ii)) completes the inductive step 95 SIMILARITIES OF CYCLIC . piecewise linear similarity implies linear equivalence for free G -representations, by using Reidemeister torsion and the Franz Independence Lemma. Topological. equivalence of G -representations im- plies linear equivalence (the case G = C(4) is trivial). Since linear G -equivalence for general finite groups G is